The present invention relates to environmentally friendly compositions and methods for removing or suppressing metal ions. More specifically, the present invention relates to competitive binders and methods of use thereof to remove or suppress metal ions from waters, flowback waters, and produced waters to make such waters suitable for subsequent use in oil-field applications. The present invention is also suitable for delinking metal cross-linked gelling agents.
Removal or suppression of interfering metal ions from water or produced water is often desirable in oil-field applications (e.g., hydraulic fracturing) because the interfering metal ions can interfere with fluid formation. Examples of interfering metal ions typically found in subterranean applications include, but are not limited to, calcium, magnesium, manganese, strontium, boron, cobalt, copper, zirconium, titanium, aluminum, iron, chromium, hafnium, and the like. These interfering metal ions can come from an external source such as ground water contamination or, in some cases, from the treatment fluids. The concentrations of interfering metal ions must generally be maintained in a suitable range, in order to reuse or dispose the spent treatment fluids.
In the absence of a method of controlling or reducing interfering metal ion concentrations, several undesirable outcomes may result. Some of these include, but are not limited to: gels not cross-linking (or not cross-linking to a suitable degree); interfering metal ions prematurely breaking the gels; interfering metal ions activating oxidizers which in turn can quickly break the gels; gels undergoing syneresis due to the presence of divalent or trivalent metal ions due to over cross-linking; and precipitates forming at elevated pH, which can cause undesirable complications such as plugging of the subterranean formation.
In fracturing applications, interfering metal ions are often introduced as cross-linking agents to coordinate the functional groups of polymeric chains found in fracturing fluids. While this is an important process to increase the viscosity of the fracturing fluids, it is often desirable to suppress or remove the interfering metal ions at a later time in order to reduce the viscosity of the fracturing fluids. For example, a lower viscosity may be desirable to allow flowback of the spent fracturing fluid, for example, as part of the fracturing fluid disposal process. This reduction in viscosity is often achieved by introducing a breaker to the fracturing fluid that breaks the cross-linking bonds and the polymer backbone of the polymeric gels.
It is also important, in fracturing applications, to control the kinetics of breaking. A breaker that acts too rapidly may hinder the proppant carrying capacity of a particular fracturing fluid. A low viscosity fluid such as water is usually inadequate for fracturing applications because its low viscosity limits its ability to transport proppant.
Currently, there are breakers that are designed to provide delayed breaking action of an oilfield fluid. However, at relatively high temperatures, these breakers tend to perform poorly (e.g., break rapidly) or degrade (e.g., enzyme breakers are susceptible to temperature dependent denaturation). Moreover, these breakers are not environmentally friendly, which may cause regulatory issues. For example, the countries of the North Sea region strongly favor the use of chemicals that have high biodegradability in seawater and low toxicity. Thus, chemicals that are used and discharged in offshore drilling operations are typically evaluated for environmental friendliness. Breakers that receive the red rating under the guidelines of The Convention for the Protection of the Marine Environment of the North East Atlantic (also known as the “OSPAR Convention”) have low degradability rates and/or may be toxic. These breakers typically require permission to discharge and/or are being phased out.